Auto body components such as doors, hoods, fenders, etc. are typically formed from two pieces of sheet metal hemmed together at the peripheral edges. Hemming involves folding the edge of one panel over the edge of the other panel. Often, the hemmed sheets are also welded, soldered, glued, sealed, bolted, riveted, etc. to provide an additional connection between the pieces of sheet metal and further seal the edge to provide structural integrity and prevent migration of dirt and moisture between the joined sheets.
Examples of bending and hemming machines and sealants for the hemmed panels are described in U.S. Pat. No. 5,647,243 (cam operated mechanical hemming machine), U.S. Pat. No. 5,783,298 (adhesive containing non-compressible beads for bonding hemmed panels), U.S. Pat. No. 6,052,887 (hemming machine incorporating a deformable hemming bead), U.S. Pat. No. 6,418,771 (interior perimeter hemming machine with nested dies), U.S. Pat. No. 6,427,510 (integrated bending and hemming machine), and U.S. Pat. No. 6,578,401 (a hemming machine having a nest adjustable via an inflatable bladder).
All mechanical hemming operations suffer from the same problem: mechanical bending produces deformities in the materials. Additionally, there are some quality issues pertaining to the relative positioning of the inner sheet material with respect to the outer sheet material. In the best case, the deformities are overcome with additional processing. In the worst case, the deformities destroy the integrity of the joint.